This invention relates to edging of ophthalmic lenses, and more particularly relates to an improved apparatus and method for edging contact lenses.
It is known in the manufacture of contact lenses that an edging process is many times required to be performed on the contact lens prior to delivery to the consumer. This is due to the various contact lens manufacturing processes which can cause the contact lens to have a thick and/or an irregular peripheral edge profile following the initial making of the lens. Some common contact lens manufacturing techniques include spin casting, lathing, and static cast molding. Edging of the irregular peripheral lens edge is often necessary to smooth and thin the lens edge so that it will glide easily over the eye when placed thereon and not cause irritation or discomfort for the wearer of the lens. Since it is usually necessary to edge every lens in the manufacturing line, the robustness and efficiency of the edging process is of utmost importance so that the lens edging process cost is minimized as much as possible without sacrificing lens quality. Thus, the time it takes to edge a single lens (the lens edging cycle time) is a critical parameter affecting production costs. Polishing of the concave (posterior) and convex (anterior) surfaces of the lens is also sometimes necessary to remove surface defects. While the invention herein is primarily directed to edging of the lens periphery which lies radially outwardly of the optical zone of the lens, it is noted that it may be useful for performing lens polishing as well.
Other common problems and concerns involved in lens edging include, but are not limited to, the following:
1) the transfer of abrasive particles to the lens during edging which can harm the lens and also need to be subsequently removed from the lens, thereby increasing production time;
2) successive wear of the abrasive component over a series of lenses inevitably causing edging variability between the group of lenses edged with a particular abrasive component;
3) the wearing down of individual abrasive components which requires intermittent removal and replacement of worn abrasive components with new abrasive components, a task which results in increased production time;
4) constraints of prior art edging apparatus which do not allow both surfaces of the lens (anterior and posterior) to be edged at the same time; and
5) edging apparatus which are at least in part operator dependent, e.g., apparatus which require an operator to place the lens on a lens holder with the lens substantially centered on the lens holder, thereby causing inevitable variation between lenses due to an operator""s inherent inability to consistently center lenses on the lens holder.
Examples of some prior art contact lens polishing and edging techniques may be seen in the following patents:
U.S. Pat. No. 4,979,337 issued to Duppstadt on Dec. 25, 1990
U.S. Pat. No. 3,971,163 issued to Dow Corning Corp. on Jul. 27, 1976
U.S. Pat. No. 3,050,909 issued to Rawstron on Aug. 28, 1962
In the ""377 patent, a polishing tool is disclosed which comprises a convex, resilient polishing head covered by a polishing cloth where the head is attached to a rotatable spindle. While the head is set rotating, the polishing head and cloth are engaged with the anterior surface of a lens to thereby polish this surface of the lens. In an alternate embodiment seen in FIGS. 6-9 thereof, a circular recess is provided inwardly adjacent the periphery of the head which provides a configuration adapted to polish and smooth the lens edge in the manner seen in FIG. 8 thereof and discussed at Col. 4, lns. 37-54 and Col. 5, lns. 45-54. It will be readily appreciated that this method of lens polishing does not address many of the concerns listed above with regards to effective lens edging. In a first aspect, it is highly dependent on operator skill in that there are no mechanical control means discussed which would assist in consistent engagement of the polishing tool with the lens. It is also not disclosed how the lens is located during polishing. Furthermore, the polishing cloth will wear over time and cause variability in lens polishing due to this wearing. The cloth will also require intermittent replacement, thereby increasing production time.
In the ""909 patent, an apparatus for polishing a lens surface is disclosed which, like the head configuration of the ""377 patent, is intended to cover substantially the entire lens surface during the polishing operation. A flexible polishing sheet P is secured in an airtight manner to a fitting Q fixed to a rotatable shaft C where fitting Q defines an air chamber P1 capable of drawing a vacuum to draw sheet P inwardly and form a concave polishing surface for polishing a convex surface. Conversely, the air chamber may be pressurized to cause sheet P to bellow outwardly and form a convex polishing surface when polishing a concave surface. Polishing is effectuated by rocking one or both of the work piece holder and/or the polishing sheet holder relative to the other. See, for example, Col. 5, ln. 9-Col. 6, ln. 13. The apparatus of the ""377 patent is directed solely to the polishing of the surfaces of a lens, and there is no discussion as to how one would polish or edge the periphery of a lens. The problem of lens variability due to wearing of the polishing sheet is also not recognized or addressed in this apparatus.
In the ""163 patent, an apparatus is disclosed for finishing a lens using an abrasive, flexible tape which is wound through a series of rollers from a tape feed reel to a tape take-up reel. The lens is held in a collet and brought into engagement with the web which is travelling from the feed reel to the take-up reel at a predetermined rate of speed (Col. 5, lns. 5-10). The web is held between a pair of guide rollers 44A and 44B and kept in tension by a spring clutch 60 (Col. 4., lns. 24-31). The purpose of the finishing operation according to the disclosure is to remove the xe2x80x9cbevatic bumpxe2x80x9d which is formed during a previous lens grinding operation which itself is not described (see Col. 1, lns. 59-end). The manner in which the lens is finished by this invention is not clearly demonstrated, although it states at Col. 2, lines 7-10 that xe2x80x9c . . . the grinding surface will substantially conform to the surface to be ground thereby increasing the possibility that the total surface will be finished without skipping any area.xe2x80x9d (emphasis added). The angularity of the tape is said to be adjustable with respect to the lens, although it is clear that the vertical orientation of the tape with respect to the lens as seen in FIG. 4 would not change since plate 66 can only pivot and move in the plane in which plate 66 lies. Plate 66 may be set oscillating as well within this same plane (see Col. 3, lns. 60-end and Col. 6, lns. 5-15). While this technique may be sufficient to remove the so-called bevatic bump from a lens, it does not appear to be able to edge a lens periphery in the same manner as contemplated by the present invention as set out more fully below.
In another known prior edging technique, a circular foam pad is set rotating and a lens set rotating on a spindle is engaged therewith to edge the lens. The lens may be passed back and forth across the radius of the pad while both the pad and lens are rotating. This technique suffers from all the disadvantages of the prior art mentioned above.
There therefore remains a need for a lens edging device and method which is able to smooth an irregular lens periphery and which solves the problems of the prior art edging devices described above.
The present invention provides a lens edging device and method which solves the problems of the prior art by providing a loose web of abrasive material against which the peripheral edge of a lens is engaged while the lens is set rotating on a lens holder. More particularly, the loose web of material is fed from a spool and secured at a point near the free end thereof. The free end of the web is allowed to dangle freely at a predetermined angle with respect to the orientation of the lens. The web is furthermore set oscillating along a vertical plane with respect to the lens. In the preferred embodiment, the free end of the web is formed into a loop. During operation, the lens periphery traverses the loop between the secured end of the loop to the free end thereof. The interaction between the loop and lens cause both the anterior and the posterior surfaces of the lens at the lens periphery to be engaged with the web. More particularly, during the initial upstroke of the web, the anterior surface of the lens at the periphery thereof is engaged with the web, and during the last part of the upstroke and the downstroke of the web, the posterior surface of the lens at the periphery is engaged with the web. During the last part of the downstroke and the initial part of the upstroke, the anterior surface of the lens is again engaged with the web, with the web cupping and riding over the lens edge as it travels from the anterior to the posterior surface of the lens edge and back again. This manner of lens edging is extremely effective at edging a lens with near-perfect and consistent results which are not attainable with the prior art methods. The present invention thus provides a lens edging device and method which solves each of the problems with the prior art methods described above.